1. Field of Invention
The present invention relates generally to audio processing and more particularly to adaptive noise suppression of an audio signal.
2. Description of Related Art
Currently, there are many methods for reducing background noise in an adverse audio environment. One such method is to use a stationary noise suppression system. The stationary noise suppression system will always provide an output noise that is a fixed amount lower than the input noise. Typically, the stationary noise suppression is in the range of 12-13 decibels (dB). The noise suppression is fixed to this conservative level in order to avoid producing speech distortion, which will be apparent with higher noise suppression.
In order to provide higher noise suppression, dynamic noise suppression systems based on signal-to-noise ratios (SNR) have been utilized. This SNR may then be used to determine a suppression value. Unfortunately, SNR, by itself, is not a very good predictor of speech distortion due to existence of different noise types in the audio environment. SNR is a ratio of how much louder speech is than noise. However, speech may be a non-stationary signal which may constantly change and contain pauses. Typically, speech energy, over a period of time, will comprise a word, a pause, a word, a pause, and so forth. Additionally, stationary and dynamic noises may be present in the audio environment. The SNR averages all of these stationary and non-stationary speech and noise. There is no consideration as to the statistics of the noise signal; only what the overall level of noise is.
In some prior art systems, an enhancement filter may be derived based on an estimate of a noise spectrum. One common enhancement filter is the Wiener filter. Disadvantageously, the enhancement filter is typically configured to minimize certain mathematical error quantities, without taking into account a user's perception. As a result, a certain amount of speech degradation is introduced as a side effect of the noise suppression. This speech degradation will become more severe as the noise level rises and more noise suppression is applied. That is, as the SNR gets lower, lower gain is applied resulting in more noise suppression. This introduces more speech loss distortion and speech degradation.
Some prior art systems invoke a generalized side-lobe canceller. The generalized side-lobe canceller is used to identify desired signals and interfering signals comprised by a received signal. The desired signals propagate from a desired location and the interfering signals propagate from other locations. The interfering signals are subtracted from the received signal with the intention of cancelling interference.
Many noise suppression processes calculate a masking gain and apply this masking gain to an input signal. Thus, if an audio signal is mostly noise, a masking gain that is a low value may be applied (i.e., multiplied to) the audio signal. Conversely, if the audio signal is mostly desired sound, such as speech, a high value gain mask may be applied to the audio signal. This process is commonly referred to as multiplicative noise suppression.